Micro-projection device with anti-speckle vibration mode

ABSTRACT

A micro-projection system for projecting light on a projection surface, comprising: at least one coherent light source ( 101 ); optical elements ( 102, 108, 109 ) in the optical path between said coherent light source and said projection surface; said optical elements including at least one reflective member ( 102 ) actuated by a drive signal for deviating light from said light source so as to scan a projected image onto said projecting surface; said optical elements including at least one vibrating element ( 102 ) actuated by a vibrating signal so as to reduce speckle onto said projecting surface. The corresponding method for reducing speckle is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, claims the benefit of andpriority to, previously filed U.S. patent application Ser. No.13/638,264 entitled “MICRO-PROJECTION DEVICE WITH ANTISPECKLE VIBRATIONMODE” filed on Nov. 28, 2012, which is a national phase application ofPCT/EP2010/055761 filed Apr. 28, 2010, the subject matter of both of theabove are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a micro-projection system comprising atleast one coherent light source, such as laser, and optical elements inthe optical path between the coherent light source and the projectionsurface. It also relates to a method for reducing speckle in suchmicro-projection systems.

BACKGROUND

Speckle is a phenomenon created with laser light sources, due to thefact that laser light is coherent. Parallels and synchronized wavefrontssimultaneously hit the projection surface. When the light hits thesurface, it creates constructive and destructive interference. The firstcategory of interference induces an image deterioration that is oftenvisible by human eye and/or by sensors. In addition to a loss of imagequality, visual comfort of the viewer may also be affected.

Several techniques are used in order to remove or reduce speckle. Inmany cases, light coherence reduction techniques are used. For instance,the light hitting the projection surface is provided from variousprojection angles. Polarized laser light hitting a depolarized film isalso used. Otherwise, illumination using various laser wavelengths mayalso be used.

Another approach consists in using vibration of the projection surface.The resulting systems are complex, expensive, and involve very specifichardware material.

WO2009/077198 describes an optical system comprising a coherent lightsource and optical elements for directing light from the source to atarget. The optical elements include at least one diffusing elementarranged to reduce a coherence volume of light from the source and avariable optical property element. A control system controls thevariable optical property element such that different speckle patternsare formed over time at the target with a temporal frequency greaterthan a temporal resolution of an illumination sensor or an eye of anobserver so that speckle contrast ratio in the observed illumination isreduced. The variable optical property element may be a deformablemirror with a vibrating thin plate or film. This solution requiresmodifying the projection system in order to integrate additionalcomponents, such as diffusing elements.

WO2007/112259 describes a system and method for reducing or eliminatingspeckle when using a coherent light source. A refracting device,comprising a birefringent material, is positioned such that therefracting device intercepts the coherent light. The refracting devicerotates, thereby causing the ordinary and/or extraordinary beams tomove. The human eye integrates the movement of the beams, reducing oreliminating laser speckle. The refracting device may include one or moreoptical devices formed of a birefringent material. Wave plates, such asa one-half wave plate, may be inserted between optical devices to causespecific patterns to be generated. Multiple optical devices having adifferent orientation of the horizontal component of the optical axismay also be used to generate other patterns. Furthermore, the refractingdevice may include an optical device having multiple sections ofdiffering horizontal components of the optical axis. This solutioninvolves a complex and expensive component, the rotating refractingdevice. Moreover, the integration of such device requires a specificglobal design.

US2009/0161196 describes a system and method for temporally varying theinterference pattern generated by a coherent light source to homogenizethe speckle pattern so that the speckle phenomenon is less observable.In accordance with an exemplary embodiment, an oscillating refractiveelement may be disposed within an optical system to create a temporallyvariable phase shift in the lights rays emanating from a coherent lightsource to eliminate static interference patterns on a light receivingelement, reducing the speckle phenomenon. This solution involves acomplex and expensive component, the oscillating refractive element.Moreover, the integration of such device requires a specific design ofthe projection system.

JP2001296503 describes a device for reducing speckle, which can decreasethe speckle pattern appearing on the illumination face to produceuniform illumination. The laser light with linearly polarized lighthaving the polarization plane rotated by 45 degrees is made incident ona first polarization beam splitter to be separated into the P polarizedlight component and S polarized light component. The P polarized lightcomponent is transmitted while the S polarized light component isreflected to equally separate the both polarized light components. The Ppolarized light component directly propagates while the S polarizedlight component is reflected by a return prism to produce an opticalpath difference longer than the coherence length from the optical pathlength of the P polarized light. Thus, incoherent laser light having twokinds of polarized light components with the optical path differencelonger than the coherence length is obtained as the output from thesecond polarization beam splitter. This arrangement involves a complexand expensive optical system.

US2009046361 describes a reflection type screen capable of suppressingeffect of external light and obtaining a wide angle of view and aforward projection system having a reflection type screen. Thereflection type screen includes a transparent prism arranged in parallelto the longitudinal direction of a base. The transparent prism has atransparent plane and a diffusion-reflection plane formed by arranging areflection film on one of the surfaces of the transparent prism.Projected light is diffused/reflected by the diffusion-reflection plane.The diffusion-reflection plane may be vibrated in order to decreasespeckle noise by laser light beams. For example, by vibrating thereflection type screen in up and down, right and left, or back andfourth direction, the diffusion-reflection plane may be vibrated.Otherwise, by varying the interval W between the base and the frontsheet by an air pump, the diffusion-reflection plane may also bevibrated. This arrangement involves a particular construction with aforward projection system having a complex screen configuration.

Thus, there is a need for a novel micro-projection system with reducedspeckle having MEMS micro-minors and MEMS components in general, that donot present the above mentioned drawbacks, namely the complexity andcosts problems caused by using specific configurations with additionalcomponents used only for speckle reduction.

SUMMARY OF THE INVENTION

A general aim of the invention is therefore to provide an improvedmethod and device for reducing or suppressing speckle in a lasermicro-projection system.

A further aim of the invention is to provide such method and device forreducing or suppressing speckle, which offers more possibilities forintegration with pre-existing laser micro-projection systems withoutrequiring important modifications.

Still another aim of the invention is to provide such method and devicefor reducing or suppressing speckle, providing efficient performances atreasonable cost.

Yet another aim of the invention is to provide such method and devicefor reducing or suppressing speckle, using components that can be fullyintegrated into a laser micro-projection device.

These aims are achieved thanks to the method for reducing speckle andthe micro-projection system defined in the claims.

There is accordingly provided a micro-projection system for projectinglight on a projection surface, comprising:

at least one coherent light source; optical elements in the optical pathbetween said coherent light source and said projection surface; saidoptical elements including at least one reflective member actuated by adrive signal for deviating light from said light source so as to scan aprojected image onto said projecting surface; said optical elementsincluding at least one vibrating element actuated by a vibrating signalso as to reduce speckle onto said projecting surface.

An optical element, provided in the optical path between the laser andthe projection surface, vibrates to avoid having all interferences atthe same place. Vibration level is substantially low (preferably lowerthan 100 Hz) and with limited amplitude, thereby avoiding any sharpnessreduction on the projected image.

The coherent light source is preferably a laser light source.

In a preferred embodiment, the vibrating element is one of saidreflective members. The reflective member is advantageously a MEMSscanning mirror.

In a further variant, MEMS mirror may be actuated with defined frequencyrelated to multiple eigen values of the mechanical motion resonantmodes.

The reflection may be provided onto one 2D minor or two 1D mirrors.

The micro-projection system advantageously comprises a plurality oflight sources and a beam combiner for combining the light beams from theplurality of sources, the reflective member being placed to deviate theoutput beam from the beam combiner. The beam combiner enables colorprojection, using multiple laser sources.

In a variant, the vibrating element is an additional optical elementprovided in the optical path. The vibrating element may be a minor. Themirror is advantageously provided with diffuse elements (such asnano-particules, or carbon nanotubes, etc) to further minimize speckle.In a further variant, the micro-vibration generator comprises a rotatingmicro-motor having a substantially rough surface.

An another variant, the vibrating element is a reflective membrane. In astill further variant, the vibrating element is a light source.

The vibrating element is advantageously actuated by a micro-vibrationgenerator. In an advantageous embodiment, the micro-vibration generatorprovides a signal superposed to the reflective member drive signal. In afurther embodiment, the vibrating element comprises a magnetic, thermal,piezo-electric, or electrostatic generation unit.

The invention also provides a method for reducing speckle in amicro-projection system adapted for projecting light on a projectionsurface, comprising:

providing a light with at least one coherent light source;

directing light from the light source to the projection surface;

actuating at least one reflective member with a drive signal fordeviating light from said light source so as to scan a projected imageonto said projecting surface;

actuating at least one vibrating element with a vibration signal so asto reduce speckle onto said projecting surface.

The method provides an anti-speckle effect for an observer eye or asensor receiving the generated illumination.

The method advantageously combines coherence length reduction bysubstantially dephasing and diffusing the light in order to homogenizelight intensity.

The signal may be a random signal or a noise. In a variant, the signalis superposed to the reflective member drive signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, features, aspects and advantages ofthe invention will become apparent from the following detaileddescription of embodiments, given by way of illustration and notlimitation with reference to the accompanying drawings, in which:

FIGS. 1A and 1B illustrate examples of MEMS micro-mirror-basedprojectors using respectively a single mirror moving along twoperpendicular axes, or two Degrees Of Freedom (2 DOF), and a projectorusing two MEMS micro-minors moving along one axis (1 DOF);

FIG. 2 illustrates an example of a micro-projection system with ascanning mirror drive unit and micro-vibration generator providingtogether a driving signal with a superposed micro-vibration signal;

FIG. 3 illustrates another example of a micro-projection system with amicro-vibration generator acting directly on a scanning minor;

FIG. 4 illustrates a further example of a micro-projection systemprovided with an additional minor or membrane connected to amicro-vibration generator;

FIG. 5 illustrates a still further example of a micro-projection systemprovided with an additional transparent optical element, connected to amicro-vibration generator;

FIG. 6 illustrates yet another example of a micro-projection system inwhich the light source or sources is/are provided with micro-vibrationgenerator;

FIG. 7 illustrates the main steps of the method for reducing speckle ina micro-projection device.

DETAILED DESCRIPTION

For clarity, as is generally the case in representation ofmicro-systems, the various figures are not drawn to scale.

Micro-Electro-Mechanical-Systems (MEMS) in the form of scanning ormoving micro-mirrors are currently being used for projection purposes.The projection can either be done using a single minor moving along twocentral and perpendicular axis (two degrees of freedom 2 DOF) as shownin FIG. 1A, or two minors moving along a central axis (one degree offreedom 1 DOF) both placed at 90 degrees one to each other as shown inFIG. 1B. In FIG. 1A, the image is created by centering a collimatedlaser beam produced by laser 101 in the middle of the 2 DOF micro-minorsurface 102 within the frame 105. The laser beam is reflected anddeviated in two directions, so as to project a scanned image on theprojection surface 104.

In FIG. 1B, the projected image is created by centering a collimatedlaser beam on the first 1 DOF micro-minor surface 102. The laser isreflected to a second 1 DOF micro-minor surface 103 with the rotationaxis placed at 90 degrees compared to the first 1 DOF micro-minor.During the actuation of the mirror(s), the collimated laser beam can bepulsed at a specific frequency to create an image with bright, dark andgrayscale parts. A monochromatic image is projected when a monochromaticlaser source is used. A multi color image can be projected when multipledifferent monochromatic laser sources are used simultaneously. In thelatter case, a known type beam combiner is preferably used.

A drive signal is used to operate the scanning minors 102, 103 in anoptimized way. This drive signal is provided by the scanning mirrordrive unit 107.

FIG. 2 illustrates an embodiment in which the micro-projection system isprovided with a scanning minor drive unit 107 and micro-vibrationgenerator 106 providing together a driving signal with a superposedmicro-vibration signal. In this first embodiment of the invention, themicro-mirror 102 is used as an anti-speckle mirror. The single 2 DOFmicro-minor is connected to a micro-vibration generator 106, providing asignal superposed to the reflective member drive signal.

The anti-speckle vibration is thus generated by this additional noise orrandom signal added to the vertical or to the horizontal, or to bothdrive signals, thus producing a slightly non-linear deflection of thereflecting mirrors, and a position of each projected pixel or lineslightly different from the expected position and of the position ofcorresponding pixels or lines in previous frames.

FIG. 3 illustrates an embodiment of a micro-projection system in which amicro-vibration generator 106 acts by direct contact with a scanningmirror 102. In this embodiment, the micro-vibration generator 106comprises either a magnetic, or thermal, or piezo-electric, orelectrostatic generation unit to provide the required micro-vibration.The generator is preferably placed in direct contact with the scanningminor to allow the produced micro-vibrations to be transmitted to themicro-mirror.

FIG. 4 illustrates a variant of the previous a micro-projection systemprovided with an additional minor or membrane 108 connected to amicro-vibration generator 106. As previously described for theembodiment of FIG. 3, the micro-vibration generator 106 comprises eithera magnetic, or thermal, or piezo-electric, or electrostatic generationunit to provide the required micro-vibration. In a variant, thegenerator 106 may comprise a rotating micro-motor having a substantiallyrough surface, thereby providing the required micro-vibration. Inanother variant, diffuse elements such as nano particles, carbonnanotubes, etc may also be provided on the mirror to further minimizespeckle.

FIG. 5 illustrates a still further example of a micro-projection systemprovided with an additional semi-transparent optical element 109,connected to a micro-vibration generator 106. The generator ispreferably placed in direct contact with the scanning mirror to allowthe produced micro-vibrations to be transmitted to the optical element.As previously described for the embodiment of FIG. 3, a magnetic, orthermal, or piezo-electric, or electrostatic generation unit to providethe required micro-vibration.

FIG. 6 illustrates yet another example of a micro-projection system inwhich the light source or sources is/are provided with micro-vibrationgenerator 106 also provided with a magnetic, or thermal, orpiezo-electric, or electrostatic generation unit to provide the requiredmicro-vibration.

All previous embodiments may also be implemented in a micro-projectionsystem comprising two 1 DOF micro-mirrors (as shown in FIG. 1B).

In all described embodiments, the micro-vibration generator 106generates movement of the reflective member between 10 to 1 mm,preferably between 50 to 500 nm and more preferably between 100 to 300nm. For an observer eye placed in a way to look at the projection image,the processed light provides an anti-speckle effect such that perceivedspeckle is reduced or suppressed. A similar effect is also provided fora sensor placed to receive the generated illumination.

Monochromatic scanning and projection can be achieved with the previousdescribed architectures using a single laser source. For colorprojection with multiple laser sources, similar assembly concepts can beapplied by specifically shaping a known beam combiner optical module.The beam combiner may be composed of multiple optical components withspecific coatings allowing to reflect certain wavelength and to transmitcertain other wavelengths. An example of architecture for colorprojection is using three light sources, typically red, green and blueto achieve the visible spectrum range. However this architecture is notlimited to three light sources and to the visible spectrum, but can beexpanded to multiple light sources and to the other part of thespectrum, which can help to achieve a wider spectral range.

The shape of the MEMS scanning micro-mirror is not limited to thegeometry presented in the Figures but can also have a circular or anelliptical shape. The described architectures can be either applied forfully or partially encapsulated MEMS scanning micro-mirror based onelectrostatic, electromagnetic, thermal and piezoelectric actuationprinciples.

FIG. 7 illustrates the main step of the method for reducing speckle in alaser micro-projection system. At step 201, one or more coherent lightsources, such as laser light sources are used to emit light. At step202, optical elements such as reflective members (for instance minors orreflective membranes), a beam splitter, a wave plate, etc, are used fordirecting light from the light source to a target such as a screen. Atstep 203, micro-vibrations are provided to at least one reflectivemember. Such vibrations may be generated according to two differentapproaches. First, a micro-vibration generator 106 is coupled to thescanning mirror drive unit 107, which sends a drive signal to thescanning mirror 102 with superposed micro-vibration signal. In a secondapproach, the micro-vibration generator 106 is mechanically connected toan optical element, such as a scanning-mirror, as shown in FIG. 3, to ananti-speckle mirror or membrane 108, as shown in FIG. 4, to ananti-speckle optical element 109, as shown in FIG. 5, or to the lightsource 101, as shown in FIG. 6. The micro-vibrations are generated inorder to provide micro-motion to the corresponding optical element,thereby resulting in an anti-speckle effect (step 204).

The process combines coherence length reduction by substantiallydephasing and diffusing the light in order to homogenize lightintensity. The vibrating signal applied to a vibrating generator may bea random signal or a noise, i.e., a signal uncorrelated and notsynchronized with the image signal or with the drive signal.

1-15. (canceled)
 16. An apparatus comprising: a plurality of opticalelements, the plurality of optical elements comprising: a reflectiveelement to receive a light beam and to reflect the light beam towards adisplay surface, the reflective element to oscillate about one or moreoscillation axes to scan the light beam to project an image onto theprojection surface; and a vibrating element to introduce noise into theoscillation of the reflective element.
 17. The apparatus of claim 16,the reflective element to receive a drive signal and to oscillate aboutthe one or more oscillation axes based at least in part on the drivesignal.
 18. The apparatus of claim 17, the vibrating element tointroduce noise into the drive signal.
 19. The apparatus of claim 18,the vibrating element to generate a speckle reduction signal and tosuperpose the speckle reduction signal onto the drive signal.
 20. Theapparatus of claim 16, wherein the reflective element is a MEMS scanningmirror.
 21. The apparatus of claim 16, wherein the vibrating element isan optical element.
 22. The apparatus of claim 16, wherein the vibratingelement is a mirror.
 23. The apparatus of claim 22, wherein the minorcomprises one or more diffuse elements.
 24. The apparatus of claim 16,wherein the vibrating element comprises a magnetic, thermal,piezo-electric, or electrostatic generation unit.
 25. The apparatus ofclaim 16, wherein the vibrating element comprises a rotating micro-motorhaving a substantially rough surface.
 26. A method for reducing specklein a projected image, the method comprising: receiving a drive signal toactuate a reflective member; oscillating the reflective member about oneor more oscillation axes, based on the drive signal, to scan a lightbeam to project an image onto a projection surface; and introducingnoise into the oscillation of the reflective member about the one ormore oscillation axes.
 27. The method of claim 26, comprising:generating a speckle reduction signal; and superposing the specklereduction signal onto the drive signal.
 28. The method of claim 27,wherein the speckle reduction signal comprises a random signal or noise.29. The method of claim 26, comprising receiving the light beam from alight source.
 30. A projection system comprising: a plurality of opticalelements, the plurality of optical elements comprising: one morereflective elements to receive a light beam and to reflect the lightbeam towards a display surface, the one or more reflective elements tooscillate about an oscillation axes to scan the light beam to project animage onto the projection surface; and a vibrating element to introducenoise into the oscillation of the one or more reflective elements. 31.The system of claim 30, the one or more reflective elements a single 2Degree of Freedom (2 DOF) micro-minor, the 2 DOF micro-minor to deflectin both a horizontal and vertical direction to oscillate about two ormore oscillation axes.
 32. The system of claim 31, the vibrating elementto introduce noise into the deflection of the reflective element in atleast one of the horizontal direction or the vertical direction.
 33. Thesystem of claim 30, the one or more reflective elements a first 1 Degreeof Freedom (DOF) micro-mirror and a second 1 DOF micro-mirror, the first1 DOF micro-mirror to deflect in a horizontal direction and the second 1DOF micro-mirror to deflect in a vertical direction.
 34. The system ofclaim 30, the vibrating element to introduce noise into the deflectionof at least one of the first 1 DOF micro-mirror or the second 1 DOFmicro-mirror.
 35. The system of claim 30, the one or more reflectiveelements to receive a drive signal and to oscillate about the one ormore oscillation axes based on the drive signal.
 36. The system of claim30, the vibrating element to generate a speckle reduction signal and tosuperpose the speckle reduction signal onto the drive signal.